Blood and other parenteral fluids are commonly stored at hypothermic temperatures in the range of about 2.degree.-10.degree. Centigrade to maintain freshness and viability. Before such fluids are infused into a patient, it is common practice to raise the influent fluid temperature to nearly normal patient temperature levels of 36.degree. to 38.degree. Centigrade. In some cases, such as open heart surgery, it is desired to maintain the patient at hypothermic temperatures for a period of time and infuse blood at substantially the same hypothermic temperature. As such infusions varying rate and temperature, it is best to have in-line blood warming devices which warm the blood as delivery is made from a blood bag to the patient to conserve blood and reduce delays of off-line heating methods. A complication in the warming of viable physiologic fluids, such as blood, is a maximum safe temperature which may be used in the heating process. It is commonly known that the maximum safe temperature which may be used is in the range of 38.degree. Centigrade. For these reasons, the safety and efficacy of imparting heat to blood and other parenteral fluids during infusion into a patient are critical parameters of an in-line heating device.
Generally, heat exchangers, not primarily meant for parenteral fluids heating, have comprised metallic or otherwise non-flexible, heat-exchanging plates, parts of which define circuitous passages for fluids in the heating pathways. U.S. Pat. Nos. 1,961,660, 2,424,792, 2,582,871, and 3,590,917 are examples of such heat exchangers. Devices which provide for heat exchange between fluids residing between juxtaposed parallel cavities are disclosed in U.S. Pat. Nos. 3,823,457, 4,258,784, and 4,744,414.
In some devices, it is important to avoid pressure buildup within a heating device, U.S. Pat. No. 3,718,182 teaches a heat exchange device wherein outlet pipes are made of a larger diametral size than the inlet pipe to reduce internal pressure buildup. Internal pressure has been used to fashion hollow metal articles using high input pressure in the forming process as disclosed in U.S. Pat. No. 2,766,514.
A plurality of physiologic fluid heaters are known in the art. Devices for heating parenteral fluids, especially blood, comprise water, air, and electrical plate modalities. As an example, U.S. Pat. No. 4,678,460 teaches a heater for parenteral fluids which comprises a water warmer and a bubble rap for eliminating air from the parenteral fluid. Another water heater which heats blood flowing through a coiled tube is taught in U.S. Pat. No. 3,614,385. U.S. Pat. No. 4,707,587 describes an air circulating heater which measures the incoming and outgoing temperatures of both blood and the heat transporting air. U.S. Pat. No. 4,759,749 discloses a sterilizable metal heat exchanger for physiological fluids.
Much of the known related art teaches heating parenteral fluids or blood in disposable devices employing tubes or bags.
As an example of a tube employing device, U.S. Pat. No. 4,532,414 discloses an in-line fluid warmer for parenteral fluids. The fluid warmer includes a box-like enclosure containing a heated plate having a sinuously-shaped groove to accept and hold a length of conduit in heat exchanging relationship. U.S. Pat. No. 4,293,762 discloses another heater comprising a serpentine pattern wherein a tube through which fluid to be heated flows.
While it is common to use a heater bag which comprises a labyrinth on serpentine path wherein the fluid is heated as it courses a relatively long pathway across heating elements. U.S. Pat. No. 4,309,592 discloses a bag and device mostly open to upwardly directed fluid flow. A restrictor is employed between an inlet pipe and main portion of the fluid path of the bag to restrict travel of the fluid introduced by the inlet pipe to maintain substantially homogeneous flow throughout the main portion of the fluid path, thereby providing more even heating of fluid within the bag. U.S. Pat. No. 4,464,563 teaches a hollow cylindrical polystyrene or polycarbonate cassette heated by two heating elements, one heating element disposed in the inner core of the cassette is heated to a higher temperature while the other heating element disposed around the outer surface of the cylinder is heated to a lower temperature.
U.S. Pat. No. 4,844,074 teaches a two stage heater for physiological fluids wherein fluid is pumped through a regulated heater into a gravity-feed container, then led by gravity flow out of the gravity-feed container through the same heater to the organism being infused. The heater comprises a bag, having a serpentine fluid path, which is disposed between heating and sensing plates.
U.S. Pat. No. 3,485,245 discloses a heater comprising two plates, each of which comprises a serpentine passageway wherein a disposable bag comprising a similar serpentine blood path is placed. U.S. Pat. No. 3,590,215 also teaches a bag or sac which comprises a serpentine pathway wherein blood is heated as it flows therethrough, but heat is supplied by pivotally mounted, thermally conductive flat plates which are in heat conductive relation with the sac. Each heating element is tapered form the inlet end to the outlet end so that the heat output diminishes in proportion to the diminishing rate of heat absorption of fluid moving from the sac inlet to the sac outlet. The shape of the heating elements are calculated to be direct functions of thermal conductivity of the sac, an important parameter due to relatively high thermal insulating qualities of plastics.
Another blood warming apparatus comprising a bag with a serpentine path is disclosed in U.S. Pat. No. 4,167,663. An apparatus is disclosed therein which comprises a control circuit responsive to the input and output temperature of the fluid being heated and heating elements which respond to signals from the control circuit. U.S. Pat. No. 4,314,143 discloses an apparatus for warming blood and other parenteral fluids through a disposable fluid system comprising a warming bag and primarily disposed to solving problems related to electrical controls and monitoring of the heated fluids.
U.S. Pat. No. 3,640,283 discloses an attachment to a plastic envelope heat sealed to form a tortuous fluid flow conduit therein. The attachment provides a drip chamber whereby air is removed as the blood flows from an outlet of the envelope.
U.S. Pat. No. 4,906,816 discloses a blood heating apparatus, comprising a box provided with a hinged door. Blood flows through a plastic pouch having a labyrinth-shaped pathway. The doors are provided with temperature controlled electrical heating assemblies which contact both sides of the pouch when the hinged door is closed, suspension pins assure position of the pouch relative to the box and door.
U.S. Pat. No. 4,680,445 discloses a heating bag disposed between modified heating plates having ridges which contour the bag thereby reducing outgassing in The bag. The bubbles which result from outgassing insulate the blood and make heating less efficient. Such outgassing is caused by the mass of the blood weighing against the bottom of each segment of the bag thereby deforming the bag and creating a negative pressure at the top of each segment of the bag where the outgassing occurs. The ridges comprise a form which reduces deformities of the bag thereby reducing the insulating bubbles.